The development of molecular probes for selective DNA imaging is of great importance for studies in cell biology and clinical diagnosis. The commercially available DNA-specific dyes generally suffer from the poor water solubility and photo bleaching issues. In addition, dyes like DAPI (4′,6-diamidino-2-phenylindole), and Hoechst have shorter life times, small stoke's shift value and require ultra-violet light illumination. Apart from these, auto fluorescence coming from the endogenous fluorophores (mitochondria, DNA and NADPH) also limit practical application of such dyes.
In this regard, Ru (II) polypyridyl complexes offer excellent luminescent properties and rich photochemistry. Their high photostablity, larger stoke's shift value (more than 150 nm) and solubility in aqueous environment makes such derivatives an ideal candidate for use as cellular imaging agent. Recently, Ru(II) complexes were used for imaging the structure of the DNA in live cells.
An article titled “A ruthenium(II) polypyridyl complex for direct imaging of DNA structure in living cells” published in NATURE CHEMISTRY, DOI: 10.1038/NCHEM.406, published in October 2009 discloses dinuclear Ru(II) complexes [(phen)2Ru(tpphz)Ru(phen)2]4+ and [(bpy)2Ru(tpphz)Ru(bpy)2]4+ wherein, phen=1,10-phenanthroline, tpphz=tetrapyrido[3,2-a: 2′,3′-c:3″,2″-h:2′″, 3′″-j]phenazine, as shown below, for use as DNA imaging agents with both luminescence and transition electron microscopy. However, these complexes are appear to be more of hydrophilic rather than lipophilic, limiting its cell diffusion across the membrane at low concentrations and therefore requires relatively in higher concentration for efficient uptake.

Another article titled “Binuclear ruthenium(II) polypyridyl complexes: DNA cleavage and mitochondria mediated apoptosis induction” published in Polyhedron, Volume 36, Issue 1, 4 Apr. 2012, Pages 45-55 discloses binuclear complexes of the type [Ru2(N—N)4(TBPhen2)]4+, where N—N=2,2′-bipyridine (bpy) (1), 1,10-phenanthroline (phen) (2), dipyrido[3,2-a:2′,3′-c]phenazine (dppz) (3) and (TBPhen2)=bis-phenanthroline Troger's Base analogue. These complexes 1& 2 are found to induce apoptosis. Therefore, the complexes studied in this article are cytotoxic and hence are not useful for DNA imaging in cells.

Another article entitled “Chiral Ruthenium (II) Polypyridyl Complexes: Stabilization of G-Quadruplex DNA, Inhibition of Telomerase Activity and Cellular Uptake” published in PLOS ONE, on December 2012, Volume 7, Issue 12, e50902, discloses Two ruthenium(II) complexes, L-[Ru(phen)2(p-HPIP)]2+ and D-[Ru(phen)2(p-HPIP)]2+ as telomerase inhibitors. These complexes are reported to have relatively higher selectivity to cancer cells than to normal cells.

Further, most of the reported Ru(II) complexes are positively charged and possess limited cell membrane permeability. This significantly restricts their potential application as cellular imaging agents. To overcome this issue of cell permeability, the common practice is of using serum free media, detergent (TritonX-100) or organic solvents such as DMSO, to make the cellular uptake more facile.
Recent reports demonstrated that polyarginine conjugated Ru(II) complexes can be targeted to the nucleus with the help of long chain polyarginine peptide. However, the preparation of polyarginine conjugated Ru(II) complexes not only involves much more synthetic effort, but also requires more incubation time. Also, the conjugation of polyarginine moiety may enhance the hydrophobicity of the Ru(II) complexes, thereby limiting their uptake to the lipid bilayer of the cell membrane. Some dinuclear Ru(II)polypyridyl complexes were also reported for cellular imaging but displays relatively poor solubility in pure water. Accordingly, DMSO has to be employed to prepare working stock solutions.
If the transition metal complexes need to be functioned as DNA imaging agents, it is required to possess low cytotoxicity and high membrane permeability in addition to high solubility and photo stability. From the above, it is evident that it is difficult to design such transition complexes that meet the requirement of the aforementioned properties.
Therefore there is a pronounced interest in the development of DNA specific probes which exhibits high water solubility, high photostablity, high membrane permeability, low toxicity and large stokes shifts to facilitate its application.